Innate immunity and cytokines in liver disease: The liver is an organ with strong innate immunity, which plays an important role in host defense against microbial infection and tumor transformation. Emerging evidence suggests that innate immunity as well as a variety of cytokines produced by innate immune cells also contribute to the pathogenesis of acute and chronic liver diseases. Our laboratory has been actively studying the role of innate immunity and its associated cytokines in liver injury and repair. During the fiscal year, we have demonstrated (1) that invariant NKT cell activation inhibits liver regeneration via IL-4- and IFN-gamma-dependent mechanisms; (2) that STAT4 knockout mice are more susceptible to concanavalin A-induced T-cell hepatitis; (3) that acute and chronic IL-22 exposure play opposing roles in controlling drug-induced liver injury. Activation of invariant natural killer T cells impedes liver regeneration by way of both IFN-&#947;- and IL-4-dependent mechanisms. Invariant natural killer T (iNKT) cells are a major subset of lymphocytes found in the liver. These cells mediate various functions, including hepatic injury, fibrogenesis, and carcinogenesis. However, the function of iNKT cells in liver regeneration remains unclear. In the present study, partial hepatectomy (PHx) was used to study liver regeneration. &#945;-Galactosylceramide (&#945;-GalCer), a specific ligand for iNKT cells, was used to induce iNKT cell activation. After PHx, two strains of iNKT cell-deficient mice, CD1d-/- and J&#945;281-/- mice, showed normal liver regeneration. Injection of &#945;-GalCer before or after PHx, which rapidly stimulated interferon-gamma (IFN-&#947;) and interleukin (IL)-4 production by iNKT cells, markedly inhibited liver regeneration. In vitro treatment with IFN-&#947; inhibited hepatocyte proliferation. In agreement with this in vitro finding, genetic disruption of IFN-&#947; or its downstream signaling molecule signal transducer and activator of transcription (STAT)1 significantly abolished the &#945;-GalCer-mediated inhibition of liver regeneration. In vitro exposure to IL-4 did not affect hepatocyte proliferation, but surprisingly, genetic ablation of IL-4 or its downstream signaling molecule STAT6 partially eliminated the inhibitory effect of &#945;-GalCer on liver regeneration. Further studies revealed that IL-4 contributed to &#945;-GalCer-induced iNKT cell expansion and IFN-&#947; production, thereby inhibiting liver regeneration. Conclusion: iNKT cells play a minor role in controlling liver regeneration after PHx under healthy conditions. Activation of iNKT cells by &#945;-GalCer induces the production of IFN-&#947;, which directly inhibits liver regeneration, and IL-4, which indirectly attenuates liver regeneration by stimulating iNKT cell expansion and IFN-&#947; production. STAT4 knockout mice are more susceptible to concanavalin A-induced T-cell hepatitis STAT4, which is activated mainly by IL-12, promotes inflammatory responses by inducing Th1 and Th2 cytokines. Recent genome-wide association studies indicate that STAT4 gene variants are associated with risk of various types of liver diseases, but how STAT4 contributes to liver disease pathogenesis remains obscure. In this study, STAT4 activation was detected in liver immune cells from patients with viral hepatitis and autoimmune hepatitis, as well as in a mouse model of concanavalin A (Con A)-induced hepatitis. Such STAT4 activation was detected mainly in T cells, natural killer T cells, and macrophages and Kupffer cells, and was diminished in Il12a(-/-) and Il12b(-/-) mice. As expected, disruption of the Stat4 gene reduced production of Th1 and Th2 cytokines, but surprisingly exacerbated Con A-induced liver injury. Similarly, disruption of Il12a or Il12b also augmented Con A-induced hepatocellular damage. Further studies showed that hepatic natural killer T (NKT) cells from Con A-treated Stat4(-/-) mice had higher levels of FasL expression and increased cytotoxicity against hepatocytes than those from Con A-treated WT mice. In vitro, blocking FasL attenuated Stat4(-/-) NKT cytotoxicity against hepatocytes. In conclusion, despite up-regulation of proinflammatory cytokines, STAT4 protects against acute T-cell hepatitis, which is mediated by direct or indirect down-regulation of FasL expression on NKT cells. Acute and Chronic Effects of IL-22 on Acetaminophen-Induced Liver Injury Acetaminophen (APAP)-induced liver injury (AILI) accounts for half of the acute liver failure cases in the United States. A better understanding of the underlying mechanisms of AILI is necessary for the development of novel antidotes. We found that pretreatment with IL-22 protected mice from APAP-mediated hepatotoxicity. The protection was dependent on STAT3, as IL-22 failed to reduce APAP hepatotoxicity in liver-specific STAT3 knockout mice. In contrast to the acute exposure to IL-22, the endogenous chronic overexpression of IL-22 in IL-22 transgenic (TG) mice or IL-22 adenovirus treatment for 6 wk resulted in a markedly increased susceptibility to AILI. Furthermore, the hepatic expression levels of cytochrome 2E1 (Cyp2E1) and Cyp1A2 were much higher in IL-22TG mice. Ablation of Cyp2E1 but not hepatic STAT3 abolished AILI and protein-adduct formation in IL-22TG mice. Finally, hepatic expression of HNF-1&#945;, a transcriptional factor that is known to control Cyp2E1 expression, was elevated in IL-22TG mice compared with wild-type mice. Upregulation of hepatic Cyp2E1 was only observed in mice with constitutive overexpression of IL-22 but not with short-term treatment with one dose of IL-22 or multiple doses of IL-22 for 2 wk. In conclusion, short-term acute IL-22 exposure protects mice against AILI through STAT3 activation; however, chronic constitutive overexpression of IL-22 exacerbates AILI by increasing Cyp2E1 and toxic reactive APAP metabolite production. These findings may not only enhance our understanding of the effects of chronic inflammation on AILI in patients with liver disease, but are also helpful to identify novel therapeutic targets for the treatment of AILI.